The importance of the acetic acid as a raw material for the manufacturing of several industrial products, such as esters, monomers, insecticides, metal salts etc., having a number of end uses, is well known.
In view of the importance of the acetic acid as a basic raw material, relevant research efforts have been carried out in the last twenty years, aiming to develop a more economical process for the industrial production thereof.
After the process involving the conversion of acetylene to acetic aldehyde and then the conversion of acetic acid, there has been adopted a process involving the conversion of ethylene to acetic aldehyde and then the reaction to acetic acid. The latter process, before the increase of the oil prices, has for a long time been the most economic process and still nowadays is used for a very high percentage of the worldwide production (about 35% of the U.S. production of acetic acid).
There is also known a process for the oxidation of saturated hydrocarbons, in liquid phase, called LPO; for this process n-butane is used in USA as the starting compound, whereas virgin naphtha is used in United Kingdom. To date, 40% of the U.S. production of acetic acid takes place through the latter process.
A process, already known since the beginning of this century, but developed as an industrial process only after 1960, also became of relevant importance: it is the carbonylation of methanol. The first industrial development has been achieved by BASF with a high pressure process, whereas by the Monsanto process it became possible to operate at low pressure. By the latter process 15% of the U.S. production is covered.
Owing to the development of the automotive industry and of the plastics industry, in the last ten years a novel raw material, theoretically useful for the liquid phase oxidation to acetic acid became more and more available. It is the butene fraction recovered from the crude processing in the cracking plants, for the production of gasoline, of Diesel oil and of various cuts and for the production of ethylene, propylene etc.
However, despite these large availability of by-products containing the butenes, to date no industrial process is known, according to which butenes are used for the production of acetic acid.
In fact, despite the attempts as carried out up to date, it has not yet been possible to develop a sufficiently selective process, permitting acid acetic to be produced, with high yields and in only one step, by the oxidation of butenes.
Of course, the critical point of such a development resides in the catalyst. In the past a process for the production of acetic acid by oxidation of butenes has been proposed by Bayer A. G.; such process however had no industrial outcome, since several drawbacks both from the technical and from the industrial point of view were present.
The technical drawbacks originated from the fact that a two step process was involved, according to which in the first step, by using a ion exchange resin as the catalyst and under pressure, butyl acetate was produced from acetic acid and butenes, the reaction product being separated from the unreacted compounds by complex fractionating steps.
In the second step, the butyl acetate was oxidized, under high temperature and pressure, to acetic acid; however, after a distillation step, one third (in moles) of the thus obtained acetic acid had to be recycled to the first step.
The economic drawbacks consisted in that, due to the complexity of the process, very high investment costs were involved: furthermore there was the added drawback of a low yield, (800 kg of butenes per metric ton of produced acetic acid, whereas the theoretical yield should be of 1712 kg of acetic acid per 800 kg of butenes); consequently this process is not industrially competitive with respect to the other processes.
Another process developed by Huels is known, comprising the oxidation of butenes in the vapour phase by a catalyst of vanadium and titanium oxides, the oxidation taking place at high pressure and at a temperature of between 180.degree. and 245.degree. C. However the yield, as acetic acid, is 46% only and a number of by-products are formed, which heavily add to the costs of the industrial operation, as in the other processes, since the separation of these by-products is necessary in order to obtain acetic acid having a commercially acceptable purity. The already mentioned LPO process would be also non competitive either with the process based on the use of ethylene as the starting compound and mainly with methanol carbonylation process.
In fact, during the oxidation, due to the rather severe reaction conditions (high pressure and temperature), a number of products, besides the acetic acid, are formed, by which the operation of the plant and particularly the fractionation step are particularly difficult and complicated.
Consequently the investment costs are very high, (for instance, in the case of the virgin naphtha 13 columns are needed) and the yield of acetic acid is not greater than 50-55%. The main purpose of the present invention is that of providing a process by which unsaturated hydrocarbons of the butene class can be used as the starting compounds, with high selectivity and yields.